In a continuous ink jet printer, a continuous jet of electrically conductive ink is expelled from a microscopic orifice in a print head to form an ink filament. The ink jet is stimulated by a periodic disturbance induced by a stimulation signal applied to the ink jet head to cause the ink jet to reliably break up into an evenly spaced series of drops. An electrode located in the vicinity of the drop break-off point is employed to induce a controlled amount of charge on the ink jet filament. The induced charge is trapped on the ink drop as it separates from the filament, and the trajectory of the ink drop is determined by the interaction between the charged drop and local electric fields. In a binary type ink jet printer, drops are either charged or not. Charged drops are deflected along a catch trajectory into an ink drop catcher and uncharged drops proceed undeflected to an ink receiving surface such as paper. In another type of continuous ink jet printer, drops are selectively deflected along a plurality of printing trajectories, or a catch trajectory, depending upon the level of charge imparted to the drops by the charging electrodes.
In such continuous ink jet printers, the length of the ink jet filament varies as a function of the amplitude of the stimulation signal applied to the ink jet print head. Variables such as ink pressure, temperature, viscosity and surface tension affect the relationship between the filament length and the stimulation amplitude. Generally, at very low stimulation amplitudes, the ink jet filament is relatively long and small satellite droplets that are produced are quickly overtaken by and assumed into the main drops. As the stimulation amplitude is further increased, the ink jet filament shortens, and the velocity of the small satellite droplets increases until a point is reached where the velocity of the small satellite droplets equals the velocity of the main ink drops. This is called the infinite satellite region, and it occurs in a relatively narrow band of stimulation amplitudes. As the stimulation amplitude is further increased, the velocity of the satellite droplets is greater than the main drops, and the satellite droplets quickly overtake and are assumed into the main drops.
A further increase in stimulation amplitude produces a satellite-free region of operation, where no satellites are formed. Past the upper end of the satellite-free region, as stimulation amplitude is further increased, the filament length reaches a minimum, and begins to lengthen again. In this region of operation, called "overdrive," satellites are once again produced. Because it is difficult to control drop charging and deflection in the presence of satellites, it is desirable to operate the ink jet print head in the satellite-free region. (It should be noted that in some ink jet printing systems infinite satellites are intentionally produced and employed to print. The present invention is not directed to such ink jet printing systems employing infinite satellites to print.) Because changes in ink temperature, and viscosity (due for example, to solvent evaporation) change the relationship between stimulation amplitude and filament length, and hence the regions of satellite production, it becomes desirable to have a means for periodically automatically adjusting the stimulation amplitude to insure that the stimulation of the ink jet print head remains in the satellite-free region.
U.S. Pat. No. 4,368,474 issued Jan. 11, 1983 to Togawa et al discloses a system for controlling stimulation amplitude in an ink jet print head. In the apparatus disclosed by Togawa et al, a narrow drop charging pulse is applied to the drop charging electrode. The narrow pulse is phase shifted through the stimulation cycle while monitoring the charge induced on the ink drops by a sensing capacitor electronically connected to the ink supply. When the phase of the charging pulse matches the phase of drop separation, a charge is induced on the sampling capacitor. The production of satellites is indicated by a relatively large charge at some phase on the capacitor indicating the phase of main drop separation, and a relatively smaller charge at another phase, indicating the phase of satellite drop separation. If charge is present on the sampling capacitor at only one phase, this indicates that the ink jet print head is being operated in the satellite-free region. The stimulation amplitude is adjusted until the measurement indicates that stimulation is occurring in the satellite-free region.
One shortcoming of this method of stimulation adjustment is that the drop charge sensing apparatus suffers from a low signal-to-noise ratio. The relatively large capacitance between the ink jet and the drop charging electrodes tends to overwhelm any signal that can be detected by the sampling capacitor connected to the grounded ink supply.
Another shortcoming is due to the fact that the measuring technique relies on phase information generated by phase shifting the narrow charging signal across the stimulation cycle. In a multiple jet ink jet print head, the technique can not be performed on all the jets simultaneously, due to the difference in drop separation phase from jet to jet. This phase difference (called the "phase defect") has been found to range between 20.degree. and 90.degree. in a multijet print head having 64 jets.
A further shortcoming of the technique is the fact that the disclosed technique does not insure that the stimulation amplitude is adjusted to a point near the middle of the range of satellite-free operation. If the adjustment results in operation near the edge of the range of satellite-free operation, slight changes in ink viscosity or temperature can cause the system to go out of adjustment and start producing satellites.
Accordingly, it is an object of the present invention to provide a method and apparatus for adjusting stimulation amplitude in an ink jet print head that is free from the shortcomings noted above.